Method of assembling a dc-dc converter

ABSTRACT

A method of assembling a DC-DC converter includes: attaching first and second discrete power stage transistor dies to a first side of a substrate, the first discrete die including a high-side power transistor and the second discrete die including a low-side power transistor electrically connected to the high-side power transistor to form an output phase of the DC-DC converter; attaching an inductor to the first side of the substrate so as to electrically connect the output phase to a metal output trace on the substrate, the inductor partly covering at least one of the first and the second discrete power stage transistor dies such that each discrete power stage transistor die that is partly covered by the inductor comprises a plurality of pins that are not covered by the inductor; and visually inspecting the plurality of pins uncovered by the inductor.

TECHNICAL FIELD

The instant application relates to DC-DC converters, and moreparticularly to optimal placement of discrete power stage components ofa DC-DC converter on a circuit board.

BACKGROUND

DC-DC converters include active and passive components, includingdiscrete power stage components such as discrete high-side and low-sidepower transistor dies, for regulating the voltage of a load such as aprocessor. Each pair of discrete high-side and low-side power transistordies forms an output phase of the DC-DC converter which is coupled tothe load by a corresponding output inductor. The components of a DC-DCconverter, including the output inductors, are attached to a printedcircuit board (PCB) together with the load. The PCB has variouselectrical pathways for electrically interconnecting the DC-DC convertercomponents, including electrically connecting the discrete power stagetransistor dies of the converter to the load. DC-DC converter powerstage dies are conventionally attached to a PCB in the same plane as theoutput inductors, increasing the size of the PCB. Also, conventionallayout design practices for PCBs further complicate such an arrangementof the DC-DC converter components.

SUMMARY

According to an embodiment of a DC-DC converter, the DC-DC convertercomprises a substrate having opposing first and second sides, a firstdiscrete power stage transistor die attached to the first side of thesubstrate and comprising a high-side power transistor, and a seconddiscrete power stage transistor die attached to the first side of thesubstrate and comprising a low-side power transistor electricallyconnected to the high-side power transistor to form an output phase ofthe DC-DC converter. The DC-DC converter further comprises an inductorattached to the first side of the substrate so as to electricallyconnect the output phase to a metal output trace on the substrate. Theinductor at least partly covers at least one of the first and the seconddiscrete power stage transistor dies.

According to an embodiment of a method of assembling a DC-DC converter,the method comprises: attaching a first discrete power stage transistordie to a first side of a substrate, the first discrete power stagetransistor die comprising a high-side power transistor; attaching asecond discrete power stage transistor die to the first side of thesubstrate, the second discrete power stage transistor die comprising alow-side power transistor electrically connected to the high-side powertransistor to form an output phase of the DC-DC converter; attaching aninductor to the first side of the substrate so as to electricallyconnect the output phase to a metal output trace on the substrate, theinductor partly covering at least one of the first and the seconddiscrete power stage transistor dies so that a plurality of pins of eachdiscrete power stage transistor die partly covered by the inductor areuncovered by the inductor; and visually inspecting the plurality of pinsuncovered by the inductor.

Those skilled in the art will recognize additional features andadvantages upon reading the following detailed description, and uponviewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The elements of the drawings are not necessarily to scale relative toeach other. Like reference numerals designate corresponding similarparts. The features of the various illustrated embodiments can becombined unless they exclude each other. Embodiments are depicted in thedrawings and are detailed in the description which follows.

FIG. 1A illustrates a top plan view of a section of a substrate such asa PCB to which power stage components of a DC-DC converter are attached,according to an embodiment.

FIG. 1B shows the substrate of FIG. 1A after the power stage componentsof the DC-DC converter are attached to the substrate.

FIG. 10 shows top, front and side views of the power stage inductorattached to the substrate shown in FIG. 1B.

FIGS. 2A through 2C illustrate bottom plan views of differentembodiments of output (phase) inductors designed to at least partlycover one or more discrete power stage transistor dies of a DC-DCconverter.

FIGS. 3A through 5A and 3B through 5B illustrate respective top-downplan views of different embodiments of a section of a substrate to whichpower stage components of a DC-DC converter are to be attached.

FIGS. 6A through 8A and 6B through 8B illustrate respective top-downplan views of additional different embodiments of a section of asubstrate to which power stage components of a DC-DC converter are to beattached.

FIGS. 9A through 11A and 9B through 11B illustrate respective top-downplan views of yet additional different embodiments of a section of asubstrate to which power stage components of a DC-DC converter are to beattached.

FIG. 12 illustrates a side perspective view of a coupled inductordesigned to at least partly cover one or more discrete power stagetransistor dies of a DC-DC converter.

DETAILED DESCRIPTION

According to embodiments described herein, a DC-DC converter includes atleast one pair of discrete power stage transistor dies. Each discretepower stage transistor die includes a high-side power transistor or alow-side power transistor, but not both transistors. As such, the powertransistors which form the power (output) stage of the DC-DC converterare disposed in separate dies. Each discrete high-side transistor dieswitchably connects the load to an input voltage of the DC-DC converter,and the corresponding discrete low-side transistor die switchablyconnects the load to ground at different periods. The discrete dies canbe bare dies i.e. unpackaged dies and have a thickness of 0.6 mm orless. In another embodiment, the discrete dies are packaged dies such asa molded die package or an open cavity die package each having athickness of 1.1 mm or less.

In either case, each pair of discrete high-side and low-side power stagetransistor dies forms an output phase of the DC-DC converter. Eachoutput phase is coupled to the load by a respective inductor. In thecase of a single pair of discrete high-side and low-side power stagetransistor dies, the DC-DC converter is a single-phase converter. In thecase of two or more pairs of discrete high-side and low-side power stagetransistor dies, the DC-DC converter is a multi-phase converter. One ormore of the discrete power stage transistor dies is at least partlycovered by the inductor electrically coupled to that phase. For eachdiscrete power stage transistor die partly covered by an inductor, aplurality of pins of that discrete die can remain uncovered by theinductor, which allows for easy visual inspection of those pins afterthe components of the DC-DC converter are attached to a substrate suchas a circuit board.

FIG. 1A illustrates a top-down plan view of a section of a substrate 100such as a PCB (printed circuit board) to which power stage components ofa DC-DC converter are to be attached. FIG. 1A illustrates the substrate100 prior to attachment of the power stage components. As such, only thefootprints of the power stage components are shown in FIG. 1A. The DC-DCconverter can be a single-phase or a multi-phase converter as explainedabove. For example, the section of the substrate 100 shown in FIG. 1Acan correspond to one phase of a single-phase or one phase of amulti-phase converter where the physical layout of each phase can beidentical or different to what is shown in FIG. 1A for the one phase.That is, the output phase layout shown in FIG. 1A can be replicated foreach phase of a multi-phase converter or the output phases can havedifferent layouts.

The power stage components of the single-phase or multi-phase DC-DCconverter include input capacitors 102, output capacitors 104, discretehigh-side and low-side power transistor dies 106, 108 for each phase ofthe DC-DC converter, and an inductor 110 for coupling each phase to aload regulated by the DC-DC converter. The pair of discrete high-sideand low-side transistor dies 106, 108 shown in FIG. 1A forms one outputphase of the DC-DC converter, which is coupled to the load by theinductor 110. A circuit schematic of the phase formed by the high-sidetransistor Q1 included in the discrete high-side transistor die 106, thelow-side transistor Q2 included in the discrete low-side transistor die108, and the inductor 110 is shown in FIG. 1A. The load can be ahigh-performance integrated circuit such as a microprocessor, graphicsprocessor, network processor, etc. or other type of integrated circuitrequiring voltage regulation such as a POL (point-of-load). The load isnot shown in FIG. 1A for ease of illustration.

The substrate 100 includes various metal traces such as copper tracesfor electrically connecting the power stage components. The metal tracesare separated from one another to prevent shorting. The metal tracesinclude an input trace 112 which is at the input voltage potential (Vin)of the DC-DC converter, an output trace 114 which is at the regulatedoutput voltage potential (Vout) of the DC-DC converter, one or moreground traces 116, 118 connected to ground, and a phase trace 120.

The input capacitors 102 of the power stage are connected between theinput trace 112 and one of the ground traces 116. The output capacitors104 of the power stage are connected between the output trace 114 andone of the ground traces 118. The input side of the discrete high-sidetransistor die 106 is connected to the input trace 112, so that thehigh-side transistor Q1 included in the discrete high-side transistordie 106 can switchably connect the load to the input voltage Vin of theDC-DC converter. The input side of the discrete low-side transistor die108 is connected to one of the ground traces 116, so that the low-sidetransistor Q2 included in the discrete low-side transistor die 108 canswitchably connect the load to ground at different periods than thehigh-side transistor Q1 is active. The output side of both discretetransistor dies 106, 108 are connected to the phase trace 120. Oneterminal of the inductor 110 is attached to the phase trace 120 and theother terminal of the inductor 110 is attached to the output trace 114,so that the output sides of the discrete transistor dies 106, 108 areelectrically connected to the output trace 114 on the substrate 100through the inductor 110.

FIG. 1B shows the substrate 100 after the power stage components of theDC-DC converter are attached to the substrate 100. According to thisembodiment, the inductor 110 completely covers both discrete power stagetransistor dies 106, 108. This means that the inductor 110 and thediscrete power stage transistor dies 106, 108 have overlappingfootprints so that when the inductor 110 is attached to the substrate100, the inductor 110 provides sufficient clearance such that bothdiscrete power stage transistor dies 106, 108 are underneath and fullyor partly covered by the inductor 110. In one embodiment, the metalground trace 116 to which the discrete low-side transistor die 108 isconnected, the input trace 112 and the phase trace 120 are eachcompletely covered by the inductor 110 as shown in FIG. 1B.

FIG. 1C shows a top view, side view and front view of the inductor 110.The inductor 110 is a single inductor according to this embodiment whichcomprises a single winding 122 wound on a core 124. The single winding122 electrically connects the phase trace 120 to the output trace 114 ofthe substrate 100. More particularly, the inductor 110 comprises a firstterminal 126 electrically connected to the phase trace 120 and thereforeto the output phase formed by the discrete high-side and low-sidetransistor dies 106, 108. A second terminal (out of view) of theinductor 110 is attached to the output trace 114. The single winding 122of the inductor 110 e.g. a copper foil winding is connected between thefirst and second terminals and forms part of the main body of theinductor 110. The main body of the inductor 110 is spaced apart from thesubstrate 100 by a gap 128. In one embodiment, the main body of theinductor 110 also comprises a plastic riser 130 for providing clearance128 under inductor 110. This clearance 128 provides the gap between theinductor 110 and the substrate 100 for accommodating at least one of thediscrete power stage transistor dies 106, 108 under the inductor 110. InFIG. 1B, both discrete power stage transistor dies 106, 108 are disposedin the gap 128 and completely covered by the inductor 110. In oneembodiment, the bottom surface of the inductor 110 facing the substrate100 is spaced apart from the top surface of each discrete power stagetransistor die 106, 108 at least partly covered by the inductor 110 byat least 0.1 mm.

FIGS. 2A through 2C illustrate bottom plan view of different embodimentsof output (phase) inductors 200, 202, 204 designed to at least partlycover one or more discrete power stage transistor dies of a DC-DCconverter. The footprint of the respective inductors 200, 202, 204 alsois shown in FIGS. 2A through 2C, the footprint corresponding to theamount of space on the substrate needed for attaching the respectiveinductors 200, 202, 204 to the substrate. In FIGS. 2A and 2B, just thetwo terminals 206/208, 210/212 of the corresponding inductor 200/202 areattached to the substrate. The terminals 208, 212 of the inductor 202shown in FIG. 2B are longer than the terminals 206, 210 of the inductor200 shown in FIG. 2A. In one embodiment, the lengths L_(T1), L_(T2) ofthe first and second terminals 208, 212 of the inductor 202 in FIG. 2Bare individually longer than the combined length (L_(DHS)+L_(DLS)) ofthe discrete high-side and low-side power stage transistor dies 106,108. The discrete high-side and low-side transistor dies 106, 108 areillustrated with respective dashed boxes in FIG. 2B to show thecorresponding die footprint in relation to the length of the inductorterminals 208, 212. The terminals 214, 216 of the inductor 204 shown inFIG. 2C are substantially shorter than the terminals 206/208, 210/212 ofthe respective inductors 200/202 shown in FIGS. 2A and 2B. Also, theinductor 204 shown in FIG. 2C has an additional post 218 for stabilizingthe inductor 204 after attachment to a substrate.

FIGS. 3A through 5A and 3B through 5B illustrate respective top-downplan views of different embodiments of a section of a substrate 300 suchas a PCB (printed circuit board) to which power stage components of aDC-DC converter are to be attached. FIGS. 3A through 5A illustrate therespective substrate sections prior to assembly of the power stagecomponents, and FIGS. 3B through 5B illustrate the same substratesections after power stage component attachment. As such, FIGS. 3Athrough 5A show only the footprints of the power stage components. TheDC-DC converter can be a single-phase or a multi-phase converter and thepower stage components of the DC-DC converter include input capacitors302, output capacitors 304, discrete high-side and low-side powertransistor dies 306, 308 for each phase of the DC-DC converter, and aninductor 310 for coupling each phase to a load regulated by the DC-DCconverter as previously explained herein.

The substrate 300 includes various metal traces such as copper tracesfor electrically connecting the power stage components. The metal tracesare separated from one another to prevent shorting. The metal tracesinclude an input trace 312 which is at the input voltage potential (Vin)of the DC-DC converter, an output trace 314 which is at the regulatedoutput voltage potential (Vout) of the DC-DC converter, one or moreground traces 316, 318 connected to ground, and a phase trace 320 alsoas previously described herein.

The input capacitors 302 of the power stage are connected between theinput trace 312 and one of the ground traces 316. The output capacitors304 of the power stage are connected between the output trace 314 andone of the ground traces 318. The discrete high-side transistor die 306is electrically connected between the input trace 312 and the phasetrace 320. The discrete low-side transistor die 308 is electricallyconnected between one of the ground traces 316 and the phase trace 320.One terminal 322 of the inductor 310 is attached to the phase trace 320and the other terminal 324 is attached to the output trace 314, so thatthe outputs of the discrete transistor dies 306, 308 are electricallyconnected to the output trace 314 on the substrate 300 through theinductor 310.

In FIG. 3B, the inductor 310 completely covers the discrete low-sidetransistor die 308 and does not cover any part of the discrete high-sidetransistor die 306. According to one embodiment, the discrete low-sidetransistor die 308 is disposed in the gap between the main body of theinductor 310 and the substrate 300 as previously described herein.

In FIG. 4B, the inductor 310 at least partly covers both discretetransistor dies 306, 308. According to one embodiment, the discretehigh-side transistor die 306 is partly covered by the inductor 310 andthe discrete low-side transistor die 308 is completely covered by theinductor 310. This way, the pins 326 of each discrete high-sidetransistor die 306, 308 which remain uncovered by the inductor 310 canbe visually inspected without interference from the inductor 310.

In FIG. 5B, the inductor 310 partly covers the discrete low-sidetransistor die 308 and does not cover any part of the discrete high-sidetransistor die 306. In both FIGS. 3B and 4B, the discrete low-sidetransistor die 308 is disposed between the terminals 322, 324 of theinductor 310. In FIG. 5B, the discrete low-side transistor die 308 isdisposed adjacent to one of the inductor terminals 322, but not betweenthe inductor terminals 322, 324. In FIGS. 3B and 5B, the phase trace 320extends under the inductor 310. In FIGS. 3B and 4B, one of the groundtraces 316 extends under the inductor 310. Also, the discrete low-sidetransistor die 308 is attached to both this ground trace 316 and thephase trace 320 in a first region of the substrate 300 disposed underthe inductor 310.

FIGS. 6A through 8A and 6B through 8B illustrate respective top-downplan views of additional different embodiments of a section of asubstrate 400 such as a PCB (printed circuit board) to which power stagecomponents of a DC-DC converter are to be attached. FIGS. 6A through 8Aillustrate the respective substrate sections prior to assembly of thepower stage components, and FIGS. 6B through 8B illustrate the samesubstrate sections after power stage component attachment. As such,FIGS. 6A through 8A show only the footprints of the power stagecomponents. The DC-DC converter can be a single-phase or a multi-phaseconverter and the power stage components of the DC-DC converter includeinput capacitors 302, output capacitors 304, discrete high-side andlow-side power transistor dies 306, 308 for each phase of the DC-DCconverter, and an inductor 310 for coupling each phase to a loadregulated by the DC-DC converter as previously explained herein.

The transistor die orientations shown in FIGS. 6A through 8A and 6Bthrough 8B are similar to those shown in FIGS. 3A through 5A and 3Bthrough 5B, respectively. Different however, one or more of thesubstrate metal traces shown in FIGS. 6A through 8A and 6B through 8Bhave different layout configurations as compared to the respectivelayouts shown in FIGS. 3A through 5A and 3B through 5B. For example inFIGS. 7A and 7B, the input trace 312 extends under the inductor 310 andthe discrete high-side transistor die 306 is attached to both the inputtrace 312 and the phase trace 120 in a second region of the substrate400 disposed under the inductor 310, the second region being differentthan the first region shown in FIGS. 4A and 4B. In FIGS. 8A and 8B, bothdiscrete transistor dies 306, 308 are disposed in the gap and adjacentto the first terminal 322 of the inductor 310. In FIGS. 5A and 5B, onlythe discrete low-side transistor die 308 is disposed in the gap andadjacent to the first terminal 322 of the inductor 310. The discretehigh-side transistor die 308 remains completely uncovered by theinductor 310 in FIG. 5B.

FIGS. 9A through 11A and 9B through 11B illustrate respective top-downplan views of yet additional different embodiments of a section of asubstrate 500 such as a PCB (printed circuit board) to which power stagecomponents of a DC-DC converter are to be attached. FIGS. 9A through 11Aillustrate the respective substrate sections prior to assembly of thepower stage components, and FIGS. 9B through 11B illustrate the samesubstrate sections after power stage component attachment. As such,FIGS. 9A through 11A show only the footprints of the power stagecomponents. The DC-DC converter can be a single-phase or a multi-phaseconverter and the power stage components of the DC-DC converter includeinput capacitors 302, output capacitors 304, discrete high-side andlow-side power transistor dies 306, 308 for each phase of the DC-DCconverter, and an inductor 310 for coupling each phase to a loadregulated by the DC-DC converter as previously explained herein.

The transistor die orientations and metal trace layouts shown in FIGS.9A through 11A and 9B through 11B are similar to those shown in FIGS. 6Athrough 8A and 6B through 8B, respectively. Different however, thediscrete low-side transistor die 308 shown in FIGS. 9A through 11A and9B through 11B have a physically larger size than the discrete high-sidetransistor die 306.

Various types of inductors can be used as the output phase inductor of aDC-DC converter and which partly covers at least one of the discretetransistor dies that form that phase. For example in FIG. 3C, a singlewinding inductor 110 is shown.

FIG. 12 shows a perspective view of a coupled inductor 600 which canreplace two or more single winding phase inductors of a DC-DC converter.The coupled inductor 600 comprises two or more separate windings (out ofview) wound on the same core (also out of view). Each separate windinghas a pair of terminals 602, 604 for attachment to corresponding metaltraces on a substrate, and is associated with one phase of a multi-phaseDC-DC converter. That is, each separate winding electrically connectsone output phase of a multi-phase DC-DC converter to the output trace ofa substrate. This way, more than one phase can be realized using asingle coupled inductor. Alternatively, one or more of the inductors cancomprise separate uncoupled windings wound on the same core, and each ofthe separate uncoupled windings electrically connects one output phaseto the output trace of the substrate. In each case, the inductor 600 atleast partly covers at least one discrete power stage transistor die ofa DC-DC converter. Based on the coupled inductor design shown in FIG.12, one or both discrete power stage transistor dies of at least twophases of a multi-phase DC-DC converter can be disposed between theterminals 602, 604 of the coupled inductor and at least partly coveredby the coupled inductor 600.

Spatially relative terms such as “under”, “below”, “lower”, “over”,“upper” and the like, are used for ease of description to explain thepositioning of one element relative to a second element. These terms areintended to encompass different orientations of the device in additionto different orientations than those depicted in the figures. Further,terms such as “first”, “second”, and the like, are also used to describevarious elements, regions, sections, etc. and are also not intended tobe limiting. Like terms refer to like elements throughout thedescription.

As used herein, the terms “having”, “containing”, “including”,“comprising” and the like are open-ended terms that indicate thepresence of stated elements or features, but do not preclude additionalelements or features. The articles “a”, “an” and “the” are intended toinclude the plural as well as the singular, unless the context clearlyindicates otherwise.

With the above range of variations and applications in mind, it shouldbe understood that the present invention is not limited by the foregoingdescription, nor is it limited by the accompanying drawings. Instead,the present invention is limited only by the following claims and theirlegal equivalents.

What is claimed is:
 1. A method of assembling a DC-DC converter, themethod comprising: attaching a first discrete power stage transistor dieto a first side of a substrate, the first discrete power stagetransistor die comprising a high-side power transistor; attaching asecond discrete power stage transistor die to the first side of thesubstrate, the second discrete power stage transistor die comprising alow-side power transistor electrically connected to the high-side powertransistor to form an output phase of the DC-DC converter; attaching aninductor to the first side of the substrate so as to electricallyconnect the output phase to a metal output trace on the substrate, theinductor partly covering at least one of the first and the seconddiscrete power stage transistor dies such that each discrete power stagetransistor die that is partly covered by the inductor comprises aplurality of pins that are not covered by the inductor; and visuallyinspecting the plurality of pins uncovered by the inductor.
 2. Themethod of claim 1, wherein the inductor is attached to the first side ofthe substrate such that the inductor partly covers both the firstdiscrete power stage transistor die and the first discrete power stagetransistor die.
 3. The method of claim 1, wherein the inductor isattached to the first side of the substrate such that one of the firstand the second discrete power stage transistor dies is partly covered bythe inductor and the other one of the first and the second discretepower stage transistor dies is at least partly covered by the inductor.4. The method of claim 1, wherein the inductor is attached to the firstside of the substrate such that one of the first and the second discretepower stage transistor dies is partly covered by the inductor and theother one of the first and the second discrete power stage transistordies is completely uncovered by the inductor.
 5. The method of claim 1,wherein the inductor is a single inductor comprising a single windingwound on a core, and wherein the single winding electrically connectsthe output phase to the metal output trace.
 6. The method of claim 1,wherein the inductor is a coupled inductor comprising separate windingswound on the same core, and wherein each of the separate windingselectrically connects one output phase to the metal output trace.
 7. Themethod of claim 1, wherein the inductor comprises a first terminal, asecond terminal, and a winding connected between the first and thesecond terminals and which forms part of a main body of the inductor,and wherein attaching the inductor to the first side of the substratecomprises: attaching the first terminal of the inductor to the outputphase; attaching the second terminal of the inductor to the metal outputtrace, wherein the main body of the inductor is spaced apart from thesubstrate by a gap, and wherein at least one of the first and the seconddiscrete power stage transistor dies is attached to the first side ofthe substrate such that the at least one of the first and the seconddiscrete power stage transistor dies is disposed in the gap.
 8. Themethod of claim 7, wherein the first and the second discrete power stagetransistor dies are attached to the first side of the substrate suchthat both the first discrete power stage transistor die and the seconddiscrete power stage transistor die are disposed in the gap.
 9. Themethod of claim 7, wherein the first and the second terminals of theinductor are individually longer than a combined length of the first andthe second discrete power stage transistor dies.
 10. The method of claim7, wherein at least one of the first and the second discrete power stagetransistor dies is attached to the first side of the substrate such thatthe at least one of the first and the second discrete power stagetransistor dies is disposed between the first and the second terminalsof the inductor.
 11. The method of claim 7, wherein the first and thesecond discrete power stage transistor dies are attached to the firstside of the substrate such that both the first discrete power stagetransistor die and the second discrete power stage transistor die aredisposed in the gap and are adjacent to the first terminal of theinductor.
 12. The method of claim 7, wherein the first and the seconddiscrete power stage transistor dies are attached to the first side ofthe substrate such that: the first discrete power stage transistor dieis connected between a metal input voltage trace on the substrate and ametal output phase trace on the substrate, the metal output phase tracebeing electrically connected to the output phase; the second discretepower stage transistor die is connected between a metal ground trace onthe substrate and the metal output phase trace; the first terminal ofthe inductor is attached to the metal output phase trace; and the metaloutput phase trace extends under the inductor.
 13. The method of claim12, wherein the metal ground trace extends under the inductor, andwherein the second discrete power stage transistor die is attached toboth the metal ground trace and the metal output phase trace in a firstregion of the substrate disposed under the inductor.
 14. The method ofclaim 13, wherein the metal input voltage trace extends under theinductor, and wherein the first discrete power stage transistor die isattached to both the metal input voltage trace and the metal outputphase trace in a second region of the substrate disposed under theinductor.
 15. The method of claim 14, wherein the inductor is attachedto the first side of the substrate such that the metal ground trace, themetal input voltage trace and the metal output phase trace are eachcompletely covered by the inductor.
 16. The method of claim 7, whereinthe main body of the inductor comprises a plastic riser which providesclearance under the inductor for accommodating at least one of the firstand the second discrete power stage transistor dies.
 17. The method ofclaim 1, wherein the inductor comprises separate uncoupled windingswound on the same core, and wherein each of the separate uncoupledwindings electrically connects one output phase to the metal outputtrace.
 18. The method of claim 1, wherein the first and the seconddiscrete power stage transistor dies are bare, unpackaged dies eachhaving a thickness of 0.6 mm or less.
 19. The method of claim 1, whereinthe first and the second discrete power stage transistor dies arepackaged dies each having a thickness of 1.1 mm or less.
 20. The methodof claim 1, wherein the DC-DC converter is a multi-phase convertercomprising a plurality of output phases, and wherein at least onediscrete power stage transistor die of each output phase is partlycovered by an inductor electrically connected to that output phase, themethod further comprising: visually inspecting the plurality of pins ofeach output phase uncovered by the inductor.
 21. The method of claim 20,wherein the plurality of output phases have an identical layout on thesubstrate.
 22. The method of claim 1, wherein the first discrete powerstage transistor die is a high-side power transistor die, wherein thesecond discrete power stage transistor die is a low-side powertransistor die electrically connected to the high-side power transistordie to form the output phase of the DC-DC converter, and wherein thehigh-side power transistor die is partly covered by the inductor so thata plurality of pins of the high-side power transistor die are notcovered by the inductor.